The invention relates to extracting sand from an area of ground which is under water. In this process, a volume of water is injected so as to form a sand/water mixture which can than be conveyed via a conveyor line. The sand/water mixture is then fed to a grading device in which sand with the desired grain size is sorted out. In this way, a pool of water with a relatively great depth is formed. However, this depth may not be too great, since the slopes must not exceed a defined maximum gradient percentage. This is because excessive slope gradients are unstable and may cause the banks to collapse.
The surplus sand can be returned to the extraction location. In the long term, a layer of returned sand is formed at the extraction location and may completely cover the ground again. A consequence of such conditions is that layers of sand of the desired composition which lie at greater depths cannot be extracted.
A further problem which emerges in the extraction of sand relates to the grading device or sizing device for collecting sand with the desired grain dimensions. Such devices are highly sensitive to fluctuations in the flow rate and the composition of the mixture supplied.
These fluctuations exhibit considerable peaks and troughs. However, even briefly feeding an excessive volume of mixture leads to the action of the grading device being disrupted. For this reason, a grading device of this nature is operated at a relatively low capacity, well beneath its maximum capacity, so that even considerable peaks in the volume of mixture cannot cause blockages.
Therefore, the object of the invention is to provide a method in which these problems do not occur, or occur to a lesser extent. This is achieved by means of a method for extracting and grading or sizing granular material, such as sand, gravel or a mixture thereof, from a volume of ground, comprising spraying a liquid into the ground, resulting in a fluidized state in the said ground, in which state granular material with a relatively high relative density and/or relatively large dimensions moves downwards, and granular material with a relatively low relative density and/or relatively small dimensions moves upwards, in such a manner that a layered state is obtained, sucking our the layer of material with a specific relative density and/or size, and feeding this material which has been sucked out to a grading device or sizing device.
Since, in the method according to the invention, the ground is locally fluidized and only a certain proportion of desired grain size of the fluidized ground material is sucked out, a fluidized liquid mixture with a relatively high relative density remains behind. A mixture of this nature has a stabilizing effect on the wails of the fluidized pit, with the result that there is no risk of the slope adjoining the pit collapsing.
It is also advantageous that the grading device or sizing device is fed with a more uniform flow rate and a material of more uniform composition, without there being excessive variations in the volume of mixture. A consequence of this is that the said device is less likely to become blocked and can be operated at a higher permissible capacity without the risk of blockage becoming excessively high.
Another beneficial effect is that any unusable fraction which is present can be comminuted during the grading operation and remains behind at the extraction location. This beneficial effect leads to the commercially extractable fraction per unit area of a concession increasing.
The method according to the invention is also suitable for extracting sand fractions which are situated beneath covering layers which are of no commercial interest.
Reference is made to the method which is known from NL-A-7402559 for washing sand, with silt constituents being removed. According to this known method, sand material is separated from silt in a fluidized pit. This method does not disclose how to carry out grading work in the sand material itself or how to feed the extracted material to a grading device or a sizing device so as to obtain an increase in capacity.